Lubricants in commercial use today are prepared from a variety of natural and synthetic base stocks admixed with various additive packages and solvents depending upon their intended application. The base stocks typically include mineral oils, highly refined mineral oils, poly alpha olefins (PAO), polyalkylene glycols (PAG), phosphate esters, silicone oils, diesters and polyol esters.
One of the most demanding lubricant applications in terms of thermal and oxidative requirements is aircraft turbine oils. Polyol esters have been commonly used as base stocks in aircraft turbine oils. Despite their inherent thermal/oxidative stability as compared with other base stock (e.g., mineral oils, polyalphaolefins, etc.), even these synthetic ester lubricants are subject to oxidative degradation and cannot be used, without further modification, for long periods of time under oxidizing conditions. It is known that this degradation is primarily due to oxidation and hydrolysis of the ester base stock.
Conventional synthetic polyol ester aircraft turbine oil formulations require the addition of antioxidants (also known as oxidation inhibitors). Antioxidants reduce the tendency of the ester base stock to deteriorate in service which deterioration can be evidenced by the products of oxidation such as sludge and varnish-like deposits on the metal surfaces, and by viscosity growth. Such antioxidants include arylamines (e.g., dioctyl phenylamine and phenylalphanaphthylamine), phosphosulfurized or sulfurized hydrocarbons, and hindered phenols (e.g., butylated hydroxyl toluene) and the like.
Frequently replacing the aircraft turbine oil or adding an antioxidant thereto to suppress oxidation increases the total cost of maintaining aircraft turbines. It would be most desirable to have an ester base stock which exhibits substantially enhanced thermal/oxidative stability compared to conventional synthetic ester base stocks, and wherein the ester base stock does not require frequent replacement due to decomposition (i.e., oxidation degradation). It would also be economically desirable to eliminate or reduce the amount of antioxidant which is normally added to such lubricant base stocks.
Upon thermal oxidative stress a weak carbon hydrogen bond is cleaved resulting in a unstable carbon radical on the ester. The role of conventional antioxidants is to transfer a hydrogen atom to the unstable carbon radical and effect a "healing" of the radical. The following equation demonstrates the effect of antioxidants (AH): EQU AH.cndot.+ROO.cndot..fwdarw.A.cndot.+ROOH
The antioxidant molecule is converted into a radical, but this radical (A.cndot.) is far more stable than that of the ester-based system. Thus, the effective lifetime of the ester is extended. When the added antioxidant is consumed, the ester radicals are not healed and oxidative degradation of the polyol ester composition occurs. One measure of relative thermal/oxidative stability well known in the art is the use of high pressure differential scanning calorimeter (HPDSC).
HPDSC has been used to evaluated the thermal/oxidative stabilities of formulated automotive lubricating oils (see J. A. Walker, W. Tsang, SAE 801383), for synthetic lubricating oils (see M. Wakakura, T. Sato, Journal of Japanese Petroluem Institute, 24 (6), pp. 383-392 (1981)) and for polyol ester derived lubricating oils (see A. Zeeman, Thermochim, Acta, 80(1984)1). In these evaluations, the time for the bulk oil to oxidize was measured which is the induction time. Longer induction times have been shown to correspond to oils having higher concentrations of antioxidants or correspond to oils having more effective antioxidants. For automotive lubricants, higher induction times have been correlated with viscosity break point times.
The use of HPDSC as described herein provides a measure of stability through oxidative induction times. A polyol ester can be blended with a constant amount of dioctyl diphenylamine which is an antioxidant. This fixed amount of antioxidant provides a constant level of protection for the polyol ester base stock against bulk oxidation. Thus oils tested in this manner with longer induction times have greater intrinsic resistance to oxidation. For the high hydroxyl esters in which no antioxidant has been added, the longer induction times reflect the greater stability of the base stock by itself and also the natural antioxidancy of the esters due to the free hydroxyl group.
The present inventors have developed a unique polyol ester composition having enhanced thermal/oxidative stability when compared to conventional synthetic polyol ester compositions. This was accomplished by synthesizing a polyol ester composition from a polyol and branched acid or branched/linear acid mixture in such a way that it has a substantial amount of unconverted hydroxyl groups. Having a highly branched polyol ester backbone permits the high hydroxyl ester to act similarly to an antioxidant such that it transfers a hydrogen atom to the unstable carbon radical which is produced when the ester molecule is under thermal oxidative stress, thereby effecting a "healing" of the radical (i.e., convert the carbon radical to a stable molecule and a stable radical). This phenomenon appears to cause the thermal/oxidative stability of the novel polyol ester composition to drastically increase, as measured by high pressure differential scanning calorimetry (HPDSC). That is, this novel polyol ester composition provides an intramolecular mechanism which is capable of scavenging alkoxides and alkyl peroxides, thereby substantially reducing the rate at which oxidative degradation can occur.
The thermal and oxidative stability which is designed into the novel polyol ester compositions of the present invention eliminates or reduces the level of antioxidant which must be added to a particular lubricant, thereby providing a substantial cost savings to lubricant manufacturers.
The present invention also provides many additional advantages which shall become apparent as described below.